Double shielded objective lens for electron beam lithography system

A double shielded objective lens has been developed for an electron beam lithography (EBL) system. The lens structure consists of outer magnetic circuit of permalloy and inner ferrite pole-piece, and the ferrite is isolated from the permalloy by a gap. This gap provides high magnetic resistance and reduces a magnetic flux from the permalloy to the ferrite. Therefore, this lens structure is expected to shield inside against outside magnetic field. Computer simulations showed that the magnetic field along the optical axis from the external magnetic field was reduced to less than 15 % in the new lens compared with the previous lens. To evaluate the shielding effect experimentally, the change in the beam position on the stage was measured when an external magnetic field was applied. The shielding ability of the new lens was 50 times as large as the previous lens for the horizontal magnetic field. The double-shield structure was proven to be effective to shield the external magnetic field. Furthermore, in order to confirm shielding ability of the practical system, the positional vibration was measured as intensity fluctuation of a back-scattered electron signal from a tungsten mark edge. The beam vibration caused by an environmental field was reduced to 40% compared with the previous system. Thus, it is clarified that the double shielded objective lens is a valuable means of improving the positional accuracy.